It is known that exhaust gases from internal combustion engines contain substances which are harmful to the environment and which can pose a threat to public health. For many years, a sustained effort has been made within the automotive industry to reduce the release to the atmosphere of harmful substances carried in exhaust gases, both by modifying the combustion process itself to give a reduced yield of harmful combustion products, and by treating the exhaust gases before their emission into the atmosphere, for example by providing a catalyst to induce chemical breakdown of the harmful constituents, particularly the oxides of nitrogen (NOx), into benign compounds.
One strategy for reducing NOx emissions, known as selective catalytic reduction or SCR, involves the introduction of a reagent comprising a reducing agent, typically a liquid ammonia source such as an aqueous urea solution, into the exhaust gas stream. The reducing agent is injected into the exhaust gas upstream of an exhaust gas catalyst, known as an SCR catalyst, typically comprising a mixture of catalyst powders such as titanium oxide, vanadium oxide and tungsten oxide immobilised on a ceramic honeycomb structure. Nitrogen oxides in the exhaust gas undergo a catalysed reduction reaction with the ammonia source on the SCR catalyst, forming gaseous nitrogen and water. An example of an SCR system is described in the Applicant's European Patent Application Publication No. EP-A-2131020, the contents of which are hereby incorporated by reference.
SCR systems typically include a reagent dosing pump for delivering reagent to the exhaust gas stream. Examples of such pumps are described in the Applicant's European Patent Application Publication No. EP-A-1878920, the contents of which are hereby incorporated by reference.
In one known reagent dosing pump, a solenoid-actuated pumping arrangement is provided to increase the pressure of the reagent, and the pump includes an atomising nozzle that receives the reagent from the pumping arrangement and delivers it from an outlet end into the exhaust gas stream. The nozzle is close-coupled to the pumping arrangement, so that the nozzle and the pumping arrangement form a single unit. The outlet end of the nozzle may be positioned directly in the exhaust gas stream, so that the pumping arrangement is located close to the outside of the exhaust pipe that conveys the exhaust gases.
It will be appreciated that, in such a case, the reagent dosing pump is exposed to the high temperatures that arise in the vicinity of the exhaust system, and so the reagent can be subjected to high temperatures, in use.
The maximum temperature at which urea-based reducing agents can be used is somewhat limited. Urea crystals tend to precipitate when the temperature of the solution is greater than approximately 70° C. Precipitation is undesirable because the precipitates can cause blockages in the delivery system, for example in the small-diameter outlets typically provided in the outlet end of the atomising nozzle. In addition, the formation of precipitates alters the concentration of the remaining solution, so that the effective quantity of ammonia delivered to the exhaust flow becomes uncertain. This could lead to inefficient catalysis and an insufficient reduction in NOx emissions.
It is therefore desirable, in many cases, to provide cooling means to cool the reagent in an SCR system and, in particular, in the reagent dosing pump, to prevent overheating of the reagent. Furthermore, when solenoid-actuated pumping arrangements are used, it is also desirable to cool the solenoid coil since the performance of solenoid actuators can decrease at high temperatures.
In some arrangements, the reagent dosing pump of an SCR system may be mounted on the exhaust pipe under the body of a vehicle. Some cooling of the exhaust gases occurs as the gases flow from the engine to the location of the reagent dosing pump, which limits to a degree the temperature to which the reagent dosing pump, and hence the reagent, is exposed. In such arrangements, sufficient cooling of the reagent dosing pump may be possible by virtue of the cooling air-flow around the reagent dosing pump, and/or by providing suitable insulating means to reduce heat transfer from the exhaust pipe to the regent dosing pump.
In other arrangements, it is desirable to locate the reagent dosing pump in the engine compartment of the vehicle. In these cases, the reagent dosing pump is exposed to higher temperatures, due to the closer proximity of the dosing pump to the engine, and it is more difficult to provide a cooling air flow to the reagent dosing pump. Accordingly, the risk of the reagent overheating in use is higher than in an under-body arrangement.
It would therefore be desirable to provide a pump assembly, suitable for use as a reagent dosing pump in an SCR system, with an improved cooling arrangement.